In a conventionally known structure of a rotational shaft for rotating a robot about a rotational shaft disposed in the vertical direction, a reducer having a hollow structure is fixed above a base of the rotational shaft, and drive power is supplied by a drive motor disposed at a position offset from the center of the rotational shaft in the horizontal direction, and is transferred to the reducer through spur gear pairs (for example, refer to PTL 1). With this structure, a space vertically penetrating near the center of the rotational shaft can be provided to facilitate handling of a linear object, such as a cable or a pipe, from the base of the rotational shaft to a robot body above the reducer.